target should be made optional in OnEventOptions

[jogold]

https://github.com/awslabs/aws-cdk/blob/60f46c830e7e27dd8d5339c53166a07174ce3a4b/packages/%40aws-cdk/aws-events/lib/on-event-options.ts#L7-L16

Making target optional allows to use the rule returned by the onXxx() in another construct that will call addTarget().

const onCommitRule = repo.onCommit('CommitEvent'); // event pattern is correctly set up
                                                   // don't want to set the target here

const other = new ComplexConstructThatCallsAddTarget(this, 'Complex', {
  rule: onCommitRule
});